Tungsten-based alloys (termed heavy alloys) are commonly used in applications such as kinetic energy penetrators, hard disk drive balance weights, nuclear and medical radiation shields, high voltage electric contacts and electrodes. These materials have one very important and desirable attribute, namely high density, which is not commonly found in other metal alloys.
For kinetic energy penetrators, generally, the higher the density of the material, the greater the desired penetration. For hard disk drive counterweights, the purpose is to concentrate the maximum possible weight in the smallest possible space so as to miniaturize the volume occupied in a disk drive. For nuclear and medical radiation shields, higher density results in higher absorption of X-rays and gamma radiation. For high voltage electric contacts and electrodes, the high melting temperature and arc erosion resistance of tungsten allow for longer life span. Thus, tungsten heavy alloys in various shapes can be used economically in many important applications. However, most of the high density materials (densities greater than 16 or 17 g/cc) such as gold, rhenium, platinum, iridium and uranium are either very expensive or extremely difficult to process.
Several tungsten heavy alloy compositions have been described in the prior art. Classic conventional alloys of tungsten-nickel-iron (e.g. U.S. Pat. No. 5,145,512, entitled "Tungsten nickel iron alloys") have been widely used in commercial and defense applications because of their unique properties of high density, high strength and high ductility. Another typical alloy is tungsten-copper (e.g. U.S. Pat. No. 5,889,220 entitled "Copper-tungsten alloys and their manufacturing methods" and U.S. Pat. No. 5,686,676 entitled "Process for making improved copper-tungsten composites") which is commonly used in electrical applications because of the special combined properties of low electrical resistivity and high arc erosion resistance.
While these alloys provide unique properties in their own right, they are either magnetic or have low electrical resistivity. These properties limit their application in the areas where magnetic properties and/or low electrical resistivity are undesirable, such as counterweight balances in disk drive actuator arms.
A routine search was performed for alloys in which the major component was tungsten and in which some iron and possibly chromium were also present. No references describing compositions that approximate those taught by the present invention were found. About the closest was U.S. Pat. No. 5,821,441 (Kawamura October 1998) which discloses an alloy having between about 80 and 97% by weight tungsten, with the remainder being nickel, cobalt, copper, and optionally iron (in concentrations up to 5%). The alloy is also prepared by sintering, its main characteristic being a high level of corrosion resistance.